This invention relates generally to solid state electronic image sensors, and more particularly relates to configurations for shuttering electronic image sensors.
There have been proposed a range of configurations for enabling electronic shuttering of image sensors such as charge-coupled device (CCD) imagers. Image sensors including electronic shutter capabilities have proven to be valuable for a wide range of imaging applications in which mechanical shuttering of an image sensor would not be sufficiently fast, such as for adaptive optics with laser guide stars. In such an application, the image sensor must be shuttered on a time scale in the microsecond regime. Focal-plane arrays of CCD imagers configured for astronomy and surveillance are also optimally controlled with electronic shuttering, because although the required shutter speeds can be relatively slow, the large detector areas of the device, as well as physical space constraints of the device, pose a serious challenge to the design of mechanical shutters. For example, there has been proposed a focal-plane array of approximately 0.64 meters in diameter for the large synoptic survey telescope (LSST). Such a scale does not easily lend itself to a mechanical shutter design.
In an electronic shutter configuration for an image sensor, there is in general provided for each pixel a buried doped layer that modifies the electrical potential beneath the pixel channel region. The doping and depth of the buried layer are adjusted so that, depending on the electrical potential applied to the gate electrode of the pixel, photogenerated electrical charge in the substrate bulk either flows freely into the channel region of the pixel, in a shutter ‘open’ state, or is re-directed away from the channel region, via control of electric fields in the substrate, in a shutter ‘closed’ state.
With this shutter design, there is conventionally included some arrangement for a shutter drain at each image sensor pixel that can drain the re-directed photogenerated charge from the bulk when the pixel is biased in the shutter ‘closed’ state and the image sensor is illuminated and therefore continuing to generate photocharge. Conventionally, shutter drain regions are provided on the front, circuit side of an image sensor, disposed in channel stop regions adjacent to pixel gate electrodes. With such a configuration, the shutter drain can be biased with an electrical voltage that produces substantial charge-collection electric fields that extend laterally beneath the pixel. These fields direct unwanted photocharge to the drain regions on the front side of the substrate where the charge is dumped to an external circuit until such time as the shutter is to be again switched to the ‘open’ state.
While this shutter drain operation is in principal quite effective, it has been found that for many applications it can be very difficult or not practical to implement an electronic shutter with shutter drains in an image sensor device. For example, the channel stops in the orthogonal-transfer CCD, or OTCCD, consist of isolated islands. Embedding a shutter drain in these islands is possible, but electrical connections must be made to every such drain, leading to a dense array of metal lines across the front circuit side of the device. More importantly, pixel gate electrodes for the pixels must be pushed aside to make room for contacts to the adjacent drain regions. This is difficult in an OTCCD because the channel stops are a convenient place to interconnect gates across neighboring pixels. As a result, pixel design and layout constraints for including a full shutter implementation, including shutter drains, in image sensors such an OTCCD can significantly limit the ability to include an electronic shutter configuration in an image sensor. But without an electronic shutter implementation, many image sensor applications cannot be successfully implemented.